1. Field of Invention
This invention relates generally to self-electrooptic effect devices and, more particularly, to the integration of waveguide self-electrooptic effect devices with other waveguides.
2. Description of the Related Art
Self-electrooptic effect devices (“SEEDs”) are optoelectronic devices that exhibit optical bistability based on the intensity of incident light. The operation of a SEED using multiple quantum well (“MQW”) material has been well described, for example, by D. A. B. Miller et al. in “Novel hybrid optically bistable switch: The quantum well self-electro-optic effect device,” Appl. Phys. Lett. 45 (1), 1 Jul. 1984. SEEDs have received attention because of their potential for use as components of optical logic circuits. In these proposed circuits, SEEDs typically implement the logic functions and the SEEDs are optically interconnected by some means, thus forming a potentially complex logic circuit.
SEEDs have two basic geometries: surface-normal SEEDs and waveguide SEEDs. In surface-normal SEEDs, the incoming light is incident normal to the surface of the SEED (and also to the semiconductor substrate carrying the SEED). Therefore, optical logic circuits based on surface-normal SEEDs typically route optical signals in the third dimension (i.e., in the out-of-plane or z-dimension if the substrate of the SEED lies in the x-y plane). For example, SEEDs can be fabricated on two separate substrates and free-space optical elements used to couple optical signals back and forth between the substrates. Alternately, turning mirrors can be used to bend the optical path from the z-direction (normal to the substrate surface) to a direction in the x-y plane (parallel to the substrate surface). Regardless of the selected approach, the basic geometry of surface-normal SEEDs complicates the interconnection task due to the out-of-plane aspect.
In contrast to surface-normal SEEDs, in waveguide SEEDs, the incoming light propagates parallel to or within the plane of the semiconductor substrate, thus reducing or eliminating the need for out-of-plane light routing elements. Waveguide SEEDs operating at 1.55 μm wavelength have been demonstrated. For example, H. C. Neitzert et al. in “Self-electro-optic effect device in waveguiding configuration as optical switch and power discriminator,” Electron. Lett. 31 (2), 19 Jan. 1995, shows SEED operation of a MQW waveguide using InGaAs/InP. Twenty quantum wells were used to form a Wannier-Stark modulator with 4:1 contrast in an all-optical hysteresis loop. Although Neitzert does not connect his SEED devices into an optical circuit, the elimination of the out-of-plane geometry overcomes the problems of interconnecting surface-normal SEEDs.
However, the in-plane interconnection of waveguide SEEDs faces other challenges. The light routing elements preferably would transport light between SEEDs and would couple light to and from SEEDs in a low loss fashion, enabling a mesh of interconnected SEEDs while reducing the presence of interference or crosstalk. In addition, the fabrication process for the light routing elements and the devices that couple these elements with the SEEDs preferably would be simple and compatible with the fabrication process for SEEDs. The general approach preferably would also be scalable to allow for the interconnection of a large number of SEEDs.
In one approach, waveguide SEEDs are butt-coupled to passive waveguides. The core of the passive waveguide is aligned with the waveguide portion of the waveguide SEED. One fabrication approach is to first grow the layers required for the SEEDs. Then, in areas where passive waveguides are desired, the active SEED material is etched away and passive epitaxial material is regrown to form a passive waveguide between active SEEDs. However, since regrowth involves the complete removal of the active material and its replacement with new material, the complexity of this method leads to a low yield of functional circuits and a corresponding increase in cost per circuit. This problem is compounded as the complexity of circuits increases, thus limiting the scalability of this approach.
An approach for integrating active waveguide SEED elements with passive waveguide interconnects is to use selective-area disordering of the MQW in the regions intended to become the passive interconnects. However, disordering does not allow for removal of doping species or for the reconfiguration of the waveguide. The impurity doping required to electrically bias the SEED typically causes increased absorptive losses of light propagation in the waveguide. In the case of even moderately complex optical circuits, these losses can quickly reach a level where little usable light remains in the circuit. Similarly, layered structures designed as SEEDs are generally not ideal waveguides for propagation of light in a circuit. Thus, a SEED optical circuit preferably would use specialized optical interconnect waveguides with low loss (i.e., low or no doping) and good mode confinement (i.e., a layer structure suited to propagation of a single optical mode of reasonable lateral extent.)
Accordingly, there is a need for approaches for coupling doped SEEDs with undoped waveguides. There is also a need for compact, high-performance optical logic circuits that use waveguide SEEDs coupled with low-loss optical interconnects.